A conventional geophone uses a permanent magnet and electric coils suspended by springs or other means such as air, oil, and the like to dampen oscillations of a mass within the geophone. A number of geophones are connected to a seismic cable, which ultimately is connected to a recording vehicle, such as a truck. A seismic signal is generated by way of an explosion or other vibration source. The vibrations create motion of the geophone which in turn causes the coil to move. The movement of the coil in the magnetic field induces a flow of electric current which is detected and recorded at the seismic truck.
A velocity geophone commonly in use today measures the velocity of the sensor casing along its sensitive axis. Typically, the measurement frequency band of such a geophone is 10-500 Hz and the amplitude range is 0.25 .mu.m/sec. to 25 mm/sec (100 dB dynamic range). Such a device uses a proof mass and spring structure to detect the motion of the sensor. The spring constant and mass are selected so that the resonant frequency is typically below 10 Hz. Below resonance, the displacement of the proof mass, relative to the sensor casing, is proportional to the acceleration of the casing. Above resonance, displacement of the proof mass is equal to the displacement of the casing.
When the frequency of the driving motion from the seismic signal is above the resonance of the device, the proof mass is effectively stationary in inertial space, while the casing moves with the driving motion. Consequently, the displacement of the case, relative to the mass, is a direct measure of the driving motion (i.e., the seismic signal).
The displacement is measured electrically by the sensing coil, which acts as the proof mass and is suspended by the spring. A magnetic circuit is provided in the casing and as the coil moves in the magnetic field, an electric current is generated that is proportional to the rate of change of flux through the coil. The rate of change of flux is directly proportional to the velocity of the coil relative to the casing. One may then derive displacement by integration of the velocity signal.
The dynamic quantity measured by a geophone depends on the method for detecting the position of the mass, relative to the casing. Using electromagnetic induction in a pick-up coil gives a velocity output. A displacement geophone uses a detection method that measures the position of the mass directly and therefore gives an output that is proportional to casing displacement.
In some circumstances, this direct measurement of displacement is desirable. If the geophysical quantity of interest is the displacement, then there are definite advantages in measuring the displacement directly, rather than deriving it by integration of the velocity.
Recent developments of fiber optic technology have also shown the benefits of the application of this technology to seismic exploration in both geophones and hydrophones, in addition to the traditional role of data transmission in the system as a whole. However, there remains a need for a geophone that measures displacement directly, and in particular using an optical fiber in the sensing apparatus.